[Torch] Fix verifier crashes on malformed global slot initializer IR

[alex1xu]

Fixes #4411.

torch-mlir-opt asserts instead of emitting a diagnostic on a malformed torch.global_slot.module_initializer. #4411 reports one case — a non-symbol slotSymNames entry — but the same root cause produces two related crashes on the same op.

The root cause is verification ordering. GlobalSlotModuleInitializerOp used hasVerifier, so its verify() runs on op entrance, before the nested torch.initialize.global_slots terminator's own invariants. The verifier reaches into that terminator (getBody()->getTerminator(), then cast<FlatSymbolRefAttr> over slotSymNames) and dereferences attributes the child verifier has not validated yet. Three parseable inputs crash:

• a non-symbol or non-flat slot name hits cast<FlatSymbolRefAttr> (TorchOps.cpp:6595); Crash in GlobalSlotModuleInitializerOp verifier due to invalid attribute type in slotSymNames #4411's [159] and a nested ref like [@a::@b] (a valid SymbolRefArrayAttr element, so the declared type did not guard it) both reach it,
• a slot/operand count mismatch indexes getSlotSymNames()[i] out of bounds (TorchOps.cpp:6634),
• a non-builtin.module parent hits cast<ModuleOp>(getParentOp()) (TorchOps.cpp:6581).

Repro:

torch.global_slot.module_initializer {
  %0 = torch.constant.int 1
  "torch.initialize.global_slots"(%0) <{slotSymNames = [159]}> : (!torch.int) -> ()
}

The fix closes each at the layer that owns the invariant:

• verify() → verifyRegions(), so the module-wide checks run on op exit, after the nested terminator is verified; the parent's casts then only run once the child's invariants hold,
• slotSymNames: SymbolRefArrayAttr → FlatSymbolRefArrayAttr, enforcing the element type declaratively (covers both [159] and [@a::@b]); the custom parser only emits flat refs, so no valid IR changes,
• add HasParent<ModuleOp>, matching the sibling torch.initialize.global_slots op.

Testing: test/Dialect/Torch/invalid.mlir adds three negative cases under -verify-diagnostics, one per fix — a nested-ref slot name (rejected by the FlatSymbolRefArrayAttr constraint), an operand/slot count mismatch (rejected by the terminator's own verifier, which now runs before the parent's per-slot loop), and a non-module parent (rejected by the HasParent trait). The existing module_initializer error tests and the GlobalizeObjectGraph / inline-global-slots / erase-module-initializer suites pass under the verifyRegions() move. No e2e test is added — this is a verifier guard, so the lit negative tests are the correct layer.

[alex1xu]

@IanWood1 @zjgarvey, would you have time to take a look when you get a chance? This is my first contribution to torch-mlir (fixing #4411). happy to loop in someone else if youre not the right reviewers. Thanks!